In conventional objectives, sharp focusing upon an object to be photographed is carried out by shifting or rotating a control element operatively coupled with a movable lens member whereby the latter is axially displaced. This lens member is often a front component of the objective, or part of such component, especially for focusing at normal distances between infinity and the so-called macro range generally defined as lying between the image-ratio limits of 1:10 and 1:1. To facilitate the taking of close-up pictures in this macro range, especially with a pancratic or varifocal objective having two fixed positive components and two movable negative components, one of these movable components (generally the second one) may be shifted while the other is held stationary; see, for example, commonly owned U.S. Pat. application Ser. No. 747,207 filed Dec. 3, 1976 by Walter Besenmatter et al, now U.S. Pat. No. 4,087,160.
With normal front-lens focusing, the usual distance scale engraved on a lens barrel will provide a sufficiently accurate reading inasmuch as--given a tolerable blur circle in the image plane--the objective will have a depth of field allowing minor positional deviations. With focusing in the macro range by the displacement of a more rearwardly located lens member, however, precise setting of that lens member becomes critical since the position of the object plane changes significantly even upon the slightest shifting of that member. To help the user find the proper setting for the taking of pictures in a predetermined object plane, e.g. for producing titles with an amateur motion-picture camera, it has heretofore been the practice to provide stops or detents (e.g. ball checks) indexing the control element in the correct position. The emplacement of the detent on the objective housing, however, involves unavoidable manufacturing tolerances which are difficult to compensate; such arrangements, therefore, are costly and of limited reliability.
Similar problems arise with conventional devices for indexing the shiftable components of a varifocal objective in a preselected position of focal-length adjustment.